The present invention relates to optical probes that can be attached to the finger, toe, or appendage of a patient. More particularly, the present invention relates to devices and methods for identifying when a probe has become dislodged from a patient.
Oximetry is the measurement of the oxygen status of blood. Early detection of low blood oxygen is critical in the medical field, for example in critical care and surgical applications, because an insufficient oxygen supply can result in brain damage and death in a matter of minutes. Pulse oximetry is a widely accepted noninvasive procedure for measuring the oxygen saturation level of arterial blood, an indicator of oxygen supply. A pulse oximetry system generally consists of a probe attached to a patient, a monitor, and a cable connecting the probe and monitor. Conventionally, a pulse oximetry probe has both red and infrared (IR) light-emitting diode (LED) emitters and a photodiode detector. The probe is typically attached to a patient""s finger or toe, or a very young patient""s foot. For a finger, the probe is configured so that the emitters project light through the fingernail, the arteries, vessels, capillaries, tissue and bone. The photodiode is positioned opposite the LED so as to detect the LED transmitted light as it emerges from the finger tissues.
The pulse oximetry monitor (pulse oximeter) determines oxygen saturation by analyzing the differential absorption by arterial blood of the two wavelengths emitted by the probe. The pulse oximeter alternately activates the probe LED emitters and reads the resulting current generated by the photodiode detector. This current is proportional to the intensity of the detected light. The pulse oximeter calculates a ratio of detected red and infrared intensities, and an arterial oxygen saturation value is empirically determined based on the ratio obtained. The pulse oximeter contains circuitry for controlling the probe, processing the probe signals and displaying the patient""s oxygen saturation and pulse rate. A pulse oximeter is described in U.S. Pat. No. 5,632,272 assigned to the assignee of the present invention.
The present invention provides a number of improvements that can be incorporated into a pulse oximeter probe to detect when a probe has become dislodged from a patient and/or to prevent a probe-off condition. A probe-off condition occurs when the optical probe becomes partially or completely dislodged from the patient, but may continue to detect an AC signal within the operating region of the pulse oximeter.
In one aspect, the present invention provides a number of electrical contacts that contact the skin of a patient when the probe is properly attached. The pulse oximeter can check the continuity through the contacts to determine whether the probe is properly attached. If the probe is not properly attached, the pulse oximeter can identify a probe-off condition even though the oximeter measures an AC signal that appears like the probe is still attached.
In another aspect, the present invention provides a number of louvers placed in front of the probe""s photodetector to filter out oblique light rays that do not originate from a point in front of the detector. If the probe becomes dislodged, the emitter will not likely remain in front of the photodetector. If the emitter and photodetector are not properly aligned, the photodetector will not produce a signal within the valid operating range of the pulse oximeter. The louvers prevent light from an oblique angle from reaching the photodetector and creating a false signal that might be interpreted by the pulse oximeter as a physiological signal. Accordingly, the pulse oximeter can determine that a probe has become dislodged when the photodetector does not produce a valid signal. Furthermore, probe-off conditions can avoided since oblique light rays are not able to reach the photodetector to produce an apparently valid signal.